First tree-level map of a continent provides proof of concept for large-scale, cost-effective annual mapping of trees and forests globally to track climate policies and investments
Copenhagen (May 2, 2023)– Scientists at the University of Copenhagen and the nonprofit organization CTrees have published a map of all the trees in Africa in a single year, providing the most accurate and complete picture of tree cover outside forests across the continent to date. Published in Nature Communications, the snapshot of trees in Africa in 2019 is the first tree-level map of any continent, offering a proof of concept for a cost-efficient system to annually measure and verify tree cover at a large scale.
The map exceeds all previous attempts to map woody vegetation across large scales and finds almost 30 percent of Africa's trees outside forests, such as in grasslands, savannahs, and croplands. The scientific breakthrough is only possible due to recent advances in remote sensing and AI-enabled data processing. To build the map, the scientists analyzed more than 200,000 satellite images at three-meter resolution from the PlanetScope satellite constellation, provided by Planet Labs PBC via an academic research license.
“Current monitoring systems are expensive, inconsistent across countries, and don’t account for trees outside forests,” said Florian Reiner, a Ph.D. fellow at the Univ. of Copenhagen and lead author of the study. “This research provides an answer to a problem that has vexed scientists and policymakers – how to consistently measure all tree and forest resources across countries on a repeated, annual basis, and at relatively low cost.”
In addition to their academic affiliations, the lead author and several co-authors are scientists with CTrees, a nonprofit launched in 2022 to provide data to governments, companies, and organizations seeking nature-based solutions to climate change.
The map accurately assesses tree cover across Africa, allowing comparisons at any scale, from individual trees to small farms and plantations, jurisdictions, and countries. In nine of 45 countries in Africa studied, the researchers found that trees outside forests make up more than half of total tree cover. For these countries – Botswana, Burkina Faso, Eritrea, Libya, Mali, Namibia, Niger, Mauritania, and Sudan – existing moderate resolution maps based on satellite data at ten to 30 meters resolution would not be useful for quantifying national woody resources.
“This research takes us one step closer to a global, tree-level dataset for measuring efforts by countries, jurisdictions, and projects to protect and restore forests and trees,” said Dr. Sassan Saatchi, CEO of CTrees and a co-author of the study. “Without a consistent, large-scale approach like the one demonstrated, it’s impossible to make accurate comparisons across countries or measure the trees outside forests – which are a substantial store of carbon in Africa and around the world.”
The paper identifies a path for consistent, repeated, and low-cost annual studies of tree cover that would reveal change over time. While satellite images are now available at less than one-meter resolution, and as precise as ten or 50 centimeters, their use and data processing is expensive, and the data may not be available throughout a given year to adequately measure change from tree planting, logging, deforestation, regrowth, and other activities. Three-meter resolution data allows a balance between accuracy and cost-effective data processing.
The continent-wide, tree-level data also helps overcome the challenge of comparing forest and other woody resources across countries that would otherwise be reported using different definitions of forests and with various, less accurate techniques. Previous medium-resolution maps have applied United Nations Food and Agriculture Organization definitions of forest, “other wooded land,” and “other land” to classify Africa’s tree cover. Compared to existing 10-meter resolution map using the FAO definition, the three-meter map finds 29 percent of trees in Africa are outside areas previously classified as tree cover.
The map is a proof of concept supporting the scientists’ goal of creating a global, tree-level map of the world’s trees, updated annually, to measure and verify nature-based solutions to climate change. The Intergovernmental Panel on Climate Change recently identified avoided deforestation, ecosystem restoration, and improved forest management as among the largest and most cost-effective climate mitigation opportunities, ranking just behind wind and solar energy deployment. However, investments in forestry, land use, and agriculture solutions have lagged behind other sectors, attracting just 2.5% of climate finance in 2019-20. The absence of high-quality data tools for measuring carbon in trees is a significant barrier to scaling up investments to reach the $400 billion in annual spending by 2050 that the IPCC says is needed for nature-based solutions.
“Annual tree-level measurement would be a significant boost for programs to protect and restore trees not just in dense forest stands but also dispersed trees in woodlands, farms, towns, and cities,” said Martin Brandt, associate professor at the Univ. of Copenhagen and a member of CTrees’ scientific leadership team. “It’s critical for the policy and carbon market communities to adopt a tree-level and large-scale data approach in order to comprehensively measure biomass, allow comparisons across areas, and verify progress over time.”
Looking ahead, the scientists plan to improve the map’s consistency, resolve issues resulting from image quality, repeat the analysis to show change over time, go beyond tree cover to track all biomass, and extend the study to provide global coverage. CTrees plans to integrate the tree-level data into its global data platform, and make it available for countries, jurisdictions, and projects to measure the performance of their programs and investments.
CTrees is a nonprofit organization that tracks carbon in every tree and forest on the planet. Led by an international team of scientists and engineers, CTrees delivers science-driven data on forest carbon to governments, companies, and organizations seeking to reduce carbon emissions from deforestation and degradation and restore forests at all scales. https://ctrees.org/
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